The incandescent lamp is the primary luminary for household and residential lighting. This lamp generally includes an incandescent filament within a predetermined and non-oxidizing atmosphere which is contained in a teardrop shaped envelope and mounted, for example, within an Edison-type base which is screwed into a permanent fixture or into a movable socket.
Despite their wide-spread use, incandescent lamps are relatively inefficient, producing only 15 to 17 lumens per watt of input power and have relatively short, unpredictable service lives. Fluorescent lamps, which have efficacies as high as 80 lumens per watt, provide an attractive alternative to incandescent lighting. Conventional fluorescent lamps, however, require a long tubular envelope which, together with the need for auxiliary ballasting equipment, has somewhat limited their acceptance in the home lighting market. Increased residential use of fluorescent illumination, with attendent savings of energy, can be achieved from the development of fluorescent lamps which are directly compatible with existing sockets and incandescent lamp fixtures.
It has been proposed, for example in U.S. Pat. No. 3,849,699 to Roche, that a relatively short fluorescent lamp with attendent ballast components be mounted directly to a screw base for operation in incandescent lamp sockets. However, when a conventional fluorescent lamp is reduced in length, the luminous efficacy is greatly reduced. The loss of efficacy in prior art, short arc fluorescent lamps has been primarily attributed to two effects: (1) the voltage drop at the lamp electrodes, and therefore power losses in the lamp, remains constant as the arc length is reduced, leaving only a small portion of the lamp input power available for light production; and (2) as the arc length is reduced, the voltage drop across the discharge column is, likewise, reduced; the lamp current must, therefore, be greatly increased to maintain the input power. The positive column efficacy decreases as a function of increasing current. Increased lamp current causes lower positive column efficacy and shorter lamp life primarily because of excessive phosphor excitation.
It is known that in certain regimes of operation, the voltage drop of a gas discharge may be increased by decreasing the gas pressure. However, lamp operation at low pressure usually causes increased sputtering of material from the lamp electrodes which, in turn, greatly shortens lamp life.
It is also known that in relatively low pressure gases, the voltage drop at hollow cathodes is significantly lower than the voltage drop at equivalent conventional lamp electrodes. Hollow cathodes are, however, often difficult to start in a fluorescent lamp environment. My U.S. Pat. No. 3,883,764 with Peter D. Johnson, describes a hybrid cathode structure which combines a conventional coated lamp electrode with a hollow cathode structure to achieve the advantages of a hollow cathode with rapid starting characeristics.